1. Field of the Invention
The present invention relates to a linear motion rolling guide unit which is applied to sliding portions of machining tools, precision processing equipment and testing equipment and which consists of a track rail with raceway grooves formed longitudinally extending on both side walls thereof, a casing straddling the track rail and having raceway grooves, and a number of rolling elements that roll between the facing raceway grooves.
2. Description of the Prior Art
In a conventional linear motion rolling guide unit, when a slider sides on a track rail (simply referred to as a rail), the seal between the slider and the rail is provided by end seals mounted at both ends of the slider and an under seal mounted to the underside of the slider.
A linear motion rolling guide unit as shown in FIG. 5 has been disclosed. FIG. 5 is a perspective view showing one example of a conventional linear motion rolling guide unit. As shown in the figure, the linear motion rolling guide unit consists mainly of a rail 1 with raceway grooves 9 formed longitudinally extending on both side walls thereof and a slider 20 slidably mounted astride the rail 1. The slider 20 is slidable relative to the rail 1 and consists of a casing 2 having raceway grooves 8 at positions facing the raceway grooves 9, a number of rolling elements or balls 4, which are trapped between the opposing raceway grooves 8, 9 to allow relative motion between the rail and the casing, and end caps 5 attached to the longitudinal ends of the casing 2, the longitudinal direction being the same as the sliding direction of the casing.
The end cap 5 is fitted with an end seal 19 that provides a seal between the rail 1 and the slider 20. The end cap 5 also has a grease nipple 18 for supplying lubricant to the sliding surface between the rail 1 and the slider 20. To prevent the balls 4 from coming off the casing 2, a ball retaining band 17 is fitted into the casing 2 so as to enclose the balls 4. The casing 2 is attached with a retainer plate 3 to reliably prevent the balls 4 from coming off the casing 2 and to seal the casing 2, the longitudinal side walls 11 of the rail 1 and the underside of the casing 2.
The slider 20 is mounted astride the rail 1 and is freely slidable on the rail because of the rolling elements 4 made up of a number of balls that circulate along the raceway grooves 9 of the rail 1. That is, the rolling elements 4 in a load region, i.e., those traveling in the raceway groove 9 of the rail 1 are led into a direction changing path which is formed in the end cap 5 and further into a return path 36 formed parallel to the raceway groove 8 in the upper part of the casing 2, so that the rolling elements 4 circulate in an endless raceway. As the balls 4 trapped between the raceway groove 8 formed in the slider 20 and the raceway groove 9 formed in the rail 1 turn under load, the slider 20 can freely move relative to the rail 1.
The retainer plate 3 is simple in shape and thus can be manufactured easily. However, the retainer plate 3 has the drawback of being easily deformed by external force when there are positioning errors of the casing 2 and the rail 1 because it is formed of a thin plate. To describe in more detail, when the slider 20 slides on the rail 1, any temperature variations and swelling by lubricant of the retainer plate 3 will deform the retainer plate 3 itself degrading its sealing performance. Further, the balls 4, the rolling elements, may make contact with the retainer plate 3, increasing the sliding resistance. Another drawback is that when the slider 20 is taken from the rail 1, the balls 4 may fall from the slider.
As a means for retaining the balls, a structure has been developed which provides a ball retaining portion to a member having a raceway groove. In a construction where the raceway groove is formed directly in the side walls of the casing, however, the retaining portion on the lower side of the raceway groove becomes an obstacle making it difficult to insert a whetstone in the raceway groove to form a raceway surface in the casing.
A retainer for the linear motion rolling guide unit that solves this problem is disclosed in the Japanese Patent Laid-Open No. 112021/1989. How the balls are retained in the linear motion rolling guide unit will be explained by referring to FIGS. 6 and 7. FIG. 6 is a cross section of an example of the conventional linear motion rolling guide unit and FIG. 7 a perspective view showing one example of the retainer plate incorporated in FIG. 6. The linear motion rolling guide unit shown in FIGS. 6 and 7 has basically the same construction and function as those of the linear motion rolling guide unit of FIG. 5 and thus identical parts are given like reference numerals.
As shown in FIGS. 6 and 7, the retainer plate 3 in the linear motion rolling guide unit consists of a sealing portion 7 protruding like a lip, a sealing portion 10 having its upper surface as a sealing surface, and a retaining portion 6 for sustaining the rolling elements in the raceway grooves, all integrally formed of the same material. In the linear motion rolling guide unit, the retainer plate 3 is put in contact with underside 12 of the casing 2 and secured thereto by screwing a screw 16 through a through-hole 15 of the retainer plate 3 and into the threaded hole in the casing 2. In this linear motion rolling guide unit, the balls 4 are kept in the casing 2 by installing them in the raceway groove 8 of the casing 2 and holding them against the raceway groove 8 by the retaining portion 6 arranged along the wall surface 22 of the casing 2 on the bottom side of the raceway groove 8 and by a retaining band 17 fixed at both ends of the casing 2. The distance between the upper edge 23 of the raceway groove 8 of the casing 2 and the top edge 14 of the retaining portion 6 of the retainer plate 3 is set larger than the diameter of the ball 4 so that the balls 4 can be fitted into the raceway groove 8. The distance between the retaining band 17 and the top edge 14 of the retaining portion 6 is set smaller than the ball diameter.
The raceway grooves 9, 8 of the rail 1 and the casing 2 are formed like a letter V, and the raceway surfaces are formed by arc surfaces to make the load capacity larger than that of flat surfaces. At the upper end surface of the raceway groove 8 in the casing 2 is formed a retaining portion 6 that projects toward the rail 1 beyond the vertical line passing the center of the ball 4. The lower end surface of the raceway groove 8 which faces the upper end surface is formed retracted from the vertical ball center line toward the casing side.
The retainer plate 3 consists of a sealing portion 7 formed as a seal lip, a sealing portion 10 that fits tightly onto the underside 12 of the casing 2, and a retaining portion 6 for holding the ball 4. The retainer plate 3 is made of such a material as plastics that has enough resiliency to provide a sealing function and also a certain level of rigidity to retain the balls 4 in the raceway groove. The retainer plate 3 can be formed by injection molding as one piece consisting of a sealing portion 7, a sealing portion 10 and a retaining portion 6.
However, with the above-mentioned linear motion rolling guide unit, since the retainer plate 3 is secured at both ends to the metal casing 2 by screws 16 and hence cannot be moved relative to the casing 2, the retainer plate 3 formed of plastics such as synthetic resin will be deformed after it is swelled by lubricating oil over a long period of use. Once the retainer plate 3 is deformed, the sealing contact condition between the underside 12 of the casing 2 and the upper surface 21 of the retainer plate 3 deteriorates. The sealing portion 7 may also deform, degrading the contact condition between the sealing portion 7 and the side surface 11 of the rail 1, lowering the sealing performance of the linear motion rolling guide unit, particularly the dust-sealing effect. At the same time, the retaining portion 6 that retains the balls 4 may also deform increasing the gap between the top edge 14 of the retaining portion 6 and the retaining band 17, so that when the slide unit is taken off the rail, the balls may not be able to be contained in the casing 2 any longer because of their own weight and may fall from the casing 2.
The Japanese Utility Model Laid-Open No. 1717/1986 discloses a linear guide equipment. This linear guide apparatus consists of: a long rail having a plurality of grooves for rolling elements formed axially therein; a slide with a pair of arms which is shaped like a saddle and has rolling element grooves facing the corresponding rolling element grooves formed in the rail; a number of rolling elements that rotate trapped between the groove in the rail and the groove in the slider; end caps mounted to both ends of the slider to form a circulating path for the rolling elements and to prevent dust from entering the rolling element portion from the ends of the slider; and dust-prevention members longer than the arms of the slider and arranged at the underside of the arms, the dust-prevention members being supported at both ends by the end caps, with one of their side edges placed in contact with the sides of the rail to prevent dust from entering the rolling element portion from below.
With the above linear guide equipment, however, the dust-prevention members arranged at the underside of the arms are supported by the end caps. Since the dust-prevention members and the end caps are made of different materials, they generally have different thermal expansions caused by temperature changes. Therefore, if the thermal expansion of the dust-prevention members is larger than that of the end caps, a deflection occurs in the dust-prevention members, causing a gap between it and the rail or between it and the underside of the arms, which in turn deteriorates the sealing effect. As a result, dust can no longer be prevented from entering the interior of the linear guide equipment.